Peristaltic pumps are used in a variety of applications in which it is desirable to convey fluid in accurately controllable quantities. Peristaltic pumps typically include a rotary portion which compels the movement of a fluid by peristaltic compression of resilient tubing containing the fluid against an arcuate rigid surface known as a pump occlusion. The roller/occlusion intersection area is typically known as the “working area” of the pump.
Imaging systems using inkjet printing have become widely known, and are often implemented using thermal inkjet technology. Such technology forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land on the medium. The printer, itself, can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate hardware and software to position the medium and expel droplets so that a desired graphic is formed on the medium. A conventional print cartridge for an inkjet type printer comprises an ink containment device and an ink-expelling apparatus, commonly known as a printhead, which heats and expels ink droplets in a controlled fashion.
In some inkjet type printers, a peristaltic pump head is used to drive multiple, resilient tubes to convey ink between the containment device and the printhead. In some pump applications, flow control is achieved simply by turning the pump off. In applications requiring more precise flow control, a valve mechanism is typically provided downstream of the pump outlet to selectively permit or prevent the flow of ink from the pump.
Whether or not a separate control valve is provided, the rollers of the peristaltic pump stop at random positions. During repeated starting and stopping of pump operation, the rollers will have stopped at positions along the entire arc of the roller/occlusion intersection, causing repeated flattening and permanent deformation of the flow area of the peristaltic tubes in the working area of the pump. Over the life of the pump, tube deformation can become so severe that it significantly alters the volumetric flow rate for a given pump motor RPM.
There are two principal remedies for severe peristaltic tube deformation. The most common solution is tube replacement, which requires removal, disassembly, repair, and replacement of the entire pump. One alternative to tube replacement is the provision of a mechanism to pull open the flattened tube. Unfortunately, pulling mechanisms are relatively complex and expensive.
It can be seen from the foregoing that the need exists for a simple, inexpensive, arrangement for reducing the effect of tube flattening in peristaltic pumps.